An autopilot system for aircraft includes, in its most general contemplation, a gyroscopic device interconnected to the aircraft flight controls via servosystems for automatically controlling flight in accordance with predetermined heading and altitude settings. In domestic or privately owned aircraft, these autopilot systems, whether of the so-called two axis or three axis variety, provide means for presetting the aircraft to any desirable heading (i.e., compass heading) and the system will automatically maintain the aircraft on the heading until affirmatively commanded to either manual control or to some other heading. Other more sophisticated systems also provide the capability of maintaining a preselected altitude.
A still further option available in certain known autopilots is the ability to be able to intercept and track the aircraft down along ground-based radio beams from a landing field, or so-called instrument landing systems (ILS) beams, for use in aiding landing of the aircraft. In this way, the aircraft is capable of automatic landing under the control of the autopilot. In explanation, the instrument landing system (ILS) provides an approach path for exact alignment and descent of an aircraft on its final approach to the landing field runway. The ground equipment typically provides two transversely spaced, highly directional, high-frequency beams emanating upwardly along a preferred slope for landing an aircraft, and, along the approach, two or more marker beacons identifying the radial distance to the landing field measured along the ground. The autopilot system in the aircraft will then, upon being properly armed, intercept the ILS beams and bring the aircraft down along the proper descent approach defined by the beams to the landing field or runway.
One difficulty which is encountered in known autopilot equipment providing glide slope interception and tracking is that a basic condition for operation is the aircraft must be maintained 60 percent under the glide slope for a predetermined minimum interval of time (e.g., 20-40 seconds) before the autopilot equipment is armed. In view of the relatively crowded conditions which obtain in airfields at the present time, the required 20-40 second waiting time may frequently be neither convenient nor practical for the pilot, and because of the traffic the pilot may have to interrupt the system and take over manually several times before he is locked onto the beam in an autopilot controlled approach.
There are several situations where it is necessary for the pilot to make a "go-around" rather than proceed on and land. For example, because of other traffic in the area, the aircraft may have to be diverted from its approach and start all over again, in which case, in the usual situation, the aircraft circles the field and comes back in for a further landing attempt along the same heading as originally. For a standard go-around using conventional autopilot systems, the pilot during the coupled approach, just has to set the unmarked pitch command wheel (controlling angle of descent and ascent) in a position which is considered will provide a proper "pitch up" or attitude in the event it is necessary to disengage that part of the autopilot system which maintains the altitude at a predetermind value. Accordingly, the pitch is not always the same and also the command wheel is susceptible to being inadvertently moved which can result in either (1) a pitch up to a higher altitude while the aircraft is at a relatively low airspeed exposing the aircraft to the possibility of stalling, or (2) pitching the aircraft to a lower altitude when the aircraft is already close to the ground with the obvious risk of crashing. Moreover, since to make a go-around the pilot must turn off the altitude maintenance part of the autopilot system in order to make the go-around, the attention of the pilot is diverted at a critical time of the approach.
Still further, many autopilot systems also have the capability of slaving the autopilot to follow a preselected radio beam frequently referred to as an omni radial or just omni (short for omnidirectional radio beam). These highly directional beams are radiated in predetermined directions away from a transmitter (e.g., at a landing field) not unlike spokes in a wheel, and, if followed, will move the aircraft along a precise map heading. When an aircraft comes into range of a desired omni, it will usually have to make a turn in order to intercept the omni, and when using an autopilot system the turn is made until the aircraft effects actual intercept at a forty-five degree angle. Of course, with a moving aircraft, this turning can encompass a relatively large area which poses no difficulty if there is no air traffic in the immediate area, or the controller has not commanded the aircraft to stay on a particular heading, or there are no other obstacles preventing the turn. However, a frequent occurrence is that air traffic control directs the pilot to stay on a particular heading mode, and the pilot must at the same time, watch for the navigation indicator needle to center (indicating that the desired omni radial is being approached) before he can switch to the navigational mode of the autopilot. During this entire changeover period, the pilot must also be watching for air traffic in the area and performing what other duties may be required in the aircraft, all of which may result in the pilot having more to do than he can conveniently (or safely) accomplish.